Cardiac pacemaker lead systems fulfill two functions. The first function is to provide an electrical conduit by which a pacemaker output pulse is delivered to stimulate the local tissue adjacent to the distal tip of the lead. The second function is to sense local, intrinsic cardiac electrical activity that takes place adjacent to the distal tip of the lead.
The advantages of providing pacing therapies to both the right and left heart chambers are well established. For example, in four chamber pacing systems, four pacing leads, typically bipolar leads, are positioned for both pacing and sensing of the respective heart chambers. To provide right side pacing and sensing, leads are implanted directly in the right atrium and/or right ventricle. To provide left side stimulation and sensing, leads are transvenously implanted in the coronary sinus region, for example, in a vein such as the great vein, the left posterior ventricular (LPV) vein, or other coronary veins, proximate the left ventricle of the heart. Such placement avoids the risks associated with implanting a lead directly within the left ventricle which can increase the potential for the formation of blood clots which may become dislodged and then carried to the brain where even a small embolism could cause a stroke.
One of the problems with cardiac pacing and sensing systems is their inability to suppress far-field electrical signals. These signals are generated by depolarizations of body tissue in areas remote from the local sensing site and are manifested as propagated voltage potential wavefronts carried to and incident upon the local sensing site. A far-field signal may comprise the intrinsic signal originating from the chamber of the heart opposite the one in which a lead electrode is located. For example, for a lead electrode implanted in the right atrium, the ventricular R-wave comprises a far-field signal whose amplitude can easily swamp the smaller P-wave signal sought to be sensed thereby making difficult the discrimination of a P-wave from the higher energy QRS complex (sometimes referred to as the R-wave).
The sensing electrode(s) detect(s) the voltages of these far-field signals and interpret them as depolarization events taking place in the local tissue when such polarizations are above the threshold sensing voltage of the system. When far-field signal voltages greater than the threshold voltage are applied to the sensed signal processing circuitry of the pulse generator or pacemaker, activation of certain pacing schemes or therapies can be erroneously triggered.
With the development of programmable, universal stimulation/sensing systems, that is, three- and four-chamber combination pacemaker-cardioverter-defibrillators, accurate sensing of cardiac signals has become even more critical, and the management, suppression and/or elimination of far-field signals is vitally important to allow appropriate device algorithms to function without being confused by the undesirable far-field signals.
U.S. Pat. No. 4,579,119 discloses a tripolar atrial pacing and sensing lead comprising a passive tip electrode and a pair of spaced-apart ring electrodes. The distal ring electrode, that is, the ring electrode positioned intermediate the tip electrode and the more proximal of the pair of ring electrodes, is at all times connected to one of the two input terminals of a sense amplifier. A multiplexer at the proximal end of the lead selectively connects the remaining electrodes to a pulse generator and to the sense amplifier. Thus, during pacing the tip electrode and the proximal ring electrode are connected to the pulse generator. During sensing, the multiplexer connects the parallel combination of the tip electrode and the proximal ring electrode to the other sense amplifier input. The placement of this tripolar lead in the right atrium is said to reduce both cross-sensing, that is, the sensing of far-field signals originating in the ventricle as well as polarization potentials which would otherwise mask the evoked response. The '119 patent requires an electrical conductor extending the entire length of the lead body for each of the electrodes and does not deal with active fixation leads, that is, those incorporating an extendable/retractable helical screw-in fixation element.
U.S. Patent Application Publication US2002/0123784A1 discloses a tripolar pacing and sensing lead including three electrodes separated by interelectrode spacings that are said to maximize both sensing and pacing activities. The electrode pair comprising a passive tip electrode and a first ring electrode provides local sensing capabilities within either the atrium or the ventricle, while the electrode pair comprising the tip electrode and a second ring electrode provides pacing capabilities. Far-field artifacts are indicated to be virtually eliminated by minimizing the distance between the two sensing electrodes to provide a tightly spaced dipole that will detect the wave front passing the electrodes without susceptibility to far-field signals. Like U.S. Pat. No. 4,579,119, the tripolar electrode arrangement of publication US2002/0123784A1 requires a separate electrical conductor running the length of the lead for each electrode and is not directed to active fixation leads.
It is also recognized that in bipolar pacing and sensing leads comprising a fixed collar electrode at the tip of the lead and a fixation element in the form of a helical screw electrode extendable and retractable relative to the lead tip, the distance between the collar electrode and the electrically active portion of the helical screw electrode should be minimized to suppress responsiveness to far-field signals. See, for example, U.S. Pat. No. 5,545,201, which is herein incorporated by reference, disclosing a partially uninsulated, electrically active helical fixation element paired with a collar electrode to provide a closely spaced dipole. Because of the development of necrotic tissue about the helical fixation element and the resulting higher pacing thresholds, a ring electrode disposed proximally of the collar electrode is often included to ensure capture. However, the added ring electrode has required an additional electrical conductor within the lead body, tending to undesirably increase the diameter of the lead.